As a method for recording digital data on optical disk media, there has commonly been employed a method of uniformizing the recording density on a recording medium by making the linear velocity constant, as seen in a compact disk (hereinafter referred to as a CD), a DVD (Digital Versatile Disk), and a DVD-RAM (Digital Versatile Disk-Random Access Memory). When reproducing a digital binary signal from a playback RF (Radio Frequency) signal which is digitally recorded by performing mark width modulation so as to make the linear recording density constant, a digital read channel method employing a PRML (Partial Response Maximum Likelihood) signal processing technique has been known as a method for realizing high playback performance independently of recording quality of digital data and signal deterioration in reproduction path. When applying the PRML signal processing, it is necessary to detect a phase of a clock component corresponding to a channel bit frequency of the playback RF signal, from a signal in which an offset component in an amplitude direction is corrected, thereby to realize phase sync pull-in for synchronization of a sampling signal. When performing high-speed playback, in order to reduce power consumption by a digital circuit that operates at a high speed, there may be used a signal synchronized with a phase of a clock component corresponding to a frequency that is half of the channel bit frequency of the playback RF signal.
Hereinafter, a description will be given of a method for detecting a digital binary signal using a signal synchronized with a phase of a clock signal corresponding to a frequency that is half of the channel bit frequency of the playback RF signal.
With reference to FIG. 17, an optical disc playback signal that is reproduced from an optical recording medium 1 by a playback means 55 is input to a preamplifier 56 to emphasize an output amplitude thereof, and thereafter, the optical disc playback signal is subjected to correction that emphasizes a high-frequency band thereof by a waveform equalization means 57. The waveform equalization means 57 comprises a filter that can arbitrarily set a boost quantity and a cut-off frequency. The output of the waveform equalization means 57 is sampled to a multiple-bit digital RF signal 6 by an analog-to-digital (hereinafter referred to as “AD”) converter 5 as a means for converting an analog signal into a digital signal, using a playback clock generated by a clock generation means 58. At this time, when codes of a digital binary signal 37 to be demodulated are those in which the minimum run length is restricted by 2, like 8-16 modulation codes used for a DVD, and MTF (Mutual Transfer Function) characteristics as optical playback characteristics are distributed within a frequency band that is shorter than about ¼ of the channel bit frequency, it is theoretically possible to demodulate the digital binary signal 37 in the case where the digital binary signal 37 is sampled by the AD converter 5 using a playback clock having a frequency component that is half of the channel bit frequency, according to the sampling theorem.
Then, the sampled multiple-bit digital RF signal 6 is input to a half rate processing offset control means 59 for half rate processing, thereby to correct an offset component in the amplitude direction, which is included in the digital RF signal 6 (refer to a description for FIG. 4 of “Disclosure of the Invention” in Japanese Published Patent Application No. 2003-36612: Patent Document 1).
On the other hand, in order to realize the PRML signal processing, it is necessary to generate, from the playback signal, a sampling signal that is synchronized with a phase of a frequency that is half of the frequency of a clock component included in the playback signal. For this purpose, in a half rate processing phase sync control means 60, a half rate processing phase error information detection means 61 detects phase error information from an output signal that is generated through the AD converter 5 and the half rate processing offset control means 59, using a signal in a normal sampling position and an interpolation signal that is obtained by restoring a signal that is missing in the time direction. Then, the clock generation means 58 performs control so that the phase of the playback clock is synchronized with the phase of a frequency that is equal to half of the clock component possessed by the playback RF signal 3, on the basis of an output signal of a loop filter 62 for smoothing the generated phase error information. Thereby, it is possible to generate, using the playback clock generated through the path from the AD converter 5 to the clock generation means 5, a multiple-bit digital RF signal 6 synchronized with the phase of a frequency that is equal to half of the clock component of the playback RF signal, thereby realizing PRML signal processing.
Next, the output signal of the half rate processing offset control means 59 is input to a half rate processing adaptive equalization means 63, and the signal is subjected partial response equalization. The partial response equalization employs a PR (a, b, b, a) system by which the waveform amplitude after the equalization is separated into five values as shown in FIG. 14(c). In FIGS. 14(b) and 14(c), white circles “◯” indicate signals obtained by performing partial response equalization on the sampling signal synchronized with the phase of the frequency that is equal to half of the clock component of the playback RF signal 3, and black circles “●” indicate signals obtained by restoring signals that are missing in the time direction, using the interpolation filter 28 that can restore Nyquist band, which filter is possessed by the half rate processing adaptive equalization means 63.
As described above, since there are various types of PRML signal processing methods according to combinations of characteristics of playback waveform and modulation codes, it is necessary to select appropriate methods for the respective recording playback systems. The half rate processing adaptive equalization means 63 comprises, for example, a finite impulse response filter for performing partial response equalization, a filter coefficient learning circuit utilizing a LMS (least mean square) algorithm for performing adaptive control to minimize an equalization error that exists in the partial response equalization output signal outputted from the finite impulse response filter, and an interpolation filter 28 capable of restoring the Nyquist band for restoring signals missing in the time direction. The equalization characteristics by the finite impulse response filter is realized by varying the filter coefficients (refer to descriptions for FIGS. 6, 10, and 11 in “Disclosure of the Invention” of Patent Document 1).
Then, data demodulation is carried out by a half rate processing maximum likelihood decoder 64 which performs decoding according to the type of partial response, using a partial response equalization signal outputted by the above-mentioned sequence of operations. The half rate processing maximum likelihood decoder 64 is a Viterbi decoder which performs decoding using a frequency that is equal to half of the channel bit frequency. The Viterbi decoder performs probability calculation according to a rule of correlation of codes that are deliberately added according to the type of partial response, and estimates a most probable sequence. However, when the processing frequency is half of the channel bit frequency, adjacent two states in state transition must be regarded as one state. For example, when the half rate processing adaptive equalization means 63 outputs the signal in the normal sampling position and the interpolation signal restored by interpolation in parallel with each other, normal data in the normal sampling position and interpolation data are input to the adjacent two states, respectively, whereby parallel processing is carried out (refer to description for FIG. 12 in “Disclosure of the Invention” of Patent Document 1).
A significant reduction in power consumption can be achieved by performing the PRML signal processing method with a frequency that is equal to half of the channel bit frequency, using the characteristics of the 8-16 modulation coding or the like. Further, since it is possible to perform offset correction control and phase sync control by restoring signals that are missing in the time direction using a linear interpolation filter or a Nyquist interpolation filter, the playback performance can be maintained.
In the conventional construction, however, when an asymmetry, i.e., an up-to-down asymmetric distortion which occurs dependently of the quality of the recorded digital data in the playback RF signal, is large, a calculation error due to the asymmetry occurs in the method of correcting an offset component in the amplitude direction by restoring data missing in the time direction by linear interpolation, thereby degrading offset correction accuracy. Thereby, the offset component remains in the PRML signal processing, and the demodulation performance of the digital binary signal is degraded. Further, in the case where the offset correction accuracy is improved by data interpolation using the Nyquist filter when the offset component is corrected, since the Nyquist interpolation processing causes an increase in the length of the feedback control loop, the control performance is degraded when high-speed feedback control is required against defects or rapid offset fluctuations. Likewise, also the partial response equalization performance is degraded because a source signal of filter coefficient learning becomes to have an offset component.
On the other hand, when employing not the PRML signal processing but a level determination method which performs binary determination at an arbitrary level, and when jitter indicating the signal quality in the playback system should be accurately detected, it is desired to perform sampling with a phase that is 180° shifted from the above-mentioned sampling phase. However, when asymmetry is large, detection of a digital binary signal and detection of jitter cannot be accurately carried out.